(1) Field of the Invention
The present invention relates to an organic electroluminescent display panel with an anti-reflective layer and its fabricating method.
(2) Description of the Related Art
Contrast ratio (CR) is an important index for the consumers to pick out and buy the display. It is defined as the brightness ratio of the lightest state to the darkest state at the same point on the display. High contrast ratio means that relatively high brightness and bright colors. Accordingly, a higher contrast ratio means that better image quality of the display. Under the circumstance, black is blacker, and white is whiter, the display can appear more colors.
The organic electroluminescent display panel is self-luminous, so it has the minimum brightness when no light emitted from it. According to the definition, if the contrast ratio is measured in a darkroom, it tends to infinity because the minimum brightness tends to zero. It makes the contrast ratio lose its significance. Therefore, the contrast ratio of the organic electroluminescent display panel should be measured in a lightroom. The way is to provide an environmental light for the organic electroluminescent display panel to measure the brightness of the light reflected by the organic electroluminescent display panel which does not emit light. Industry standards currently allow for 500 lux maintained for the lightroom.
One way of obtaining the better contrast ratio is to reduce the environmental brightness. Usually, the organic electroluminescent display panel uses the black matrix to reduce the reflection in order to increase the contrast ratio.
Refer to FIG. 1A, the conventional organic electroluminescent display panel 10 includes an organic light emitting diode 11 and its driving transistor 12, which are placed in an active area 13. The organic light emitting diode 11 has a bottom electrode 111, a top electrode 112 and an organic emissive layer 113 sandwiched between the two electrode 111, 112. The driving transistor 12 includes a source metal 121, a gate metal 122, a drain metal 123 and a channel 124. The channel 124 is isolated from the gate metal 122 by an inner layer dielectric 125. The channel 124 has a source contacting area 1241 and a drain contacting area 1242 to respectively contact with the source metal 121 and the drain metal 123. The drain metal 123 is electrically connected the bottom electrode 111 of the organic light emitting diode 11.
A part of the active region 13 is covered by a black matrix pattern to form a black matrix region 131, the other part is not covered to form an opening region 132. As shown, a patterning black matrix 15 is formed on the substrate 14 in the black matrix region 131. The driving transistor 12 is disposed on the black matrix 15. Between the driving transistor 12 and the black matrix 15 has a buffer layer 17, such as silicon oxide. The opening region 132 is below the organic light emitting diode 11 to allow the light to exit, and is covered by a color filter layer 18 to define the light color. The edge of the color filter layer 18 has a black photoresist 19 corresponding to the black matrix 15.
It is note that, there is no anti-reflective structure formed between the substrate 14 and the organic light emitting diode 11 in the opening region 132, so that the reflectivity of the whole display panel is lager than 20% not to rise the contrast ratio efficiently.
FIG. 1B is a diagram showing the relation between the reflectivity and the area ratio of the black matrix region to the active region. The ordinate is the reflectivity of the organic electroluminescent display panel in the light of 550 nm. The abscissa is the area percent of the black matrix region 131 to the active region 13. FIG. 1B shows that the reflectivity decreases in proportion to the area percent of the black matrix region 131 to the active region 13. For example, when the black matrix region 131 covers the 20% of the active region 13, the reflectivity is about 60%. If the black matrix region 131 covers the active region 13 to reach to 68%, the reflectivity reduces to 25%.
Still refer to FIG. 1A, if the area ratio of the black matrix region 131 to the active region 13 is constant, an external anti-reflective film 16 is adhered to the outside of a light-emitting surface of the organic electroluminescent display panel 10 to reduce the reflectivity. However, the thickness of the organic electroluminescent display panel 10 is increased and the light transmission of the opening region 132 is reduced, so the brightness is reduced.